The theory that Malaysia Air Flight MH370 was hijacked has encouraged relatives of passengers who are holding out hope that the missing plane landed somewhere.

"My gut feeling is that it landed. I still feel his spirit. I don't feel he is dead," said Sarah Bajc, a 48-year-old American teacher living in Beijing whose partner, Philip Wood, a 50-year-old IBM executive, was a passenger on the flight.

A former technology executive, Bajc has been one of the most proactive of the family members, setting up Facebook and Twitter accounts encouraging people to keep looking for the plane.

The family of missing Australians Bob and Cathy Lawton said the prospect of hijacking was frightening.

Bob's brother David said he felt "empty" not knowing what happened to the Queensland couple.

"If they found the wreckage of the plane then that would be finalised because there's no hope. But while you've got hope, you've got worries too. Because if they're alive, are they being treated well, or what's happening?" he said.

"One part, I'm feeling maybe they're still alive and there's hope there. The other thing is, we don't know where they are. So I just don't know how to feel really."

The Malaysian government said on Sunday that it was searching over a wide area in Asia for the Boeing 777 and the 239 people it carried.

Satellite intelligence has suggested two possible routes - one to the northwest with possible destinations of Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, Pakistan, Bangladesh, India, China, Myanmar, Laos, Vietnam or Thailand, and another to the south over the Indian Ocean between Indonesia and Australia.

"We are looking at large tracts of land. We are crossing 11 countries and as well as deep and remote oceans," said Malaysia's acting transportation minister, Hishamuddin Hussein, said last night at a news conference in Kuala Lumpur.

The flight departed Kuala Lumpur for Beijing at 12.20am local time March 8 and disappeared from civilian radar within 50 minutes. However, Inmarsat satellites picked up tracking information suggesting it remained in flight until at least 8.11am.

Aviation geeks using airport data from X-Plane, a flight simulator website, have identified more than 600 runways within range of the nearly 3,000 miles that the plane could have travelled from Kuala Lumpur.

The flight carried 227 passengers, 159 of them Chinese citizens. On Chinese websites, many people voiced hope that the plane and its passengers might be found unharmed.

"It is the first time it is good news that a plane was hijacked," wrote one microblogger on the popular Sina Weibo site.

"There's still hope for my daughter and her husband to be alive," the parents of one young woman told the Beijing News.

The problem with the hijacking theory is that no group has come forward to take credit for the airplane's disappearance or to make demands.

"That makes it very difficult for us to verify if it is a hijacking or a terrorist act," Hishamuddin said.

Malaysian officials said police had searched the home of 53-year-old pilot Zaharie Adman Shah and removed a flight simulator he kept at home. The co-pilot, Farq Ab Hamid, 27, was also under investigation, as were potentially all of the passengers.

That person switched off the Aircraft Communications Addressing and Reporting System (ACARS) and transponder soon after takeover and made a sharp westward turn during a 10-minute leg of the flight between Malaysian and Vietnamese airspace, where there is little primary radar coverage.

As the flight zigzagged off course, it also rose to 45,000 feet (13,716 metres), well above the approved altitude for a Boeing 777, possibly to ensure that passengers could not use their cellphones or to incapacitate them by causing a shortage of oxygen, experts say.

"I am making the assumption at this point that the hostages are going to be leveraged," said Bajc, who said she also believed Wood would be a particularly valuable hostage because he was the only US citizen, other than two toddlers, among the passengers.

"This was clearly an orchestrated effort to take the plane. Why would anybody go to hide themselves if they were going to crash the plane into the water or commit suicide?" Bajc said.

The couple last spoke on Friday evening March 7, as Wood was getting ready to leave for the Kuala Lumpur airport for the red-eye flight back to Beijing. She had sent a car to pick him up at Beijing International Airport at 7.30am.

Shortly before 8am, the driver telephoned her to say that the flight hadn't arrived and that there was no information on the arrival board indicating a delay.

"Have you deplaned yet?" she texted him.

When she didn't hear back, she telephoned his Malaysian cellphone and his Beijing cellphone. The former went directly to voice mail and the latter gave a message saying the user had the power off.